Wave Particle Duality
De Broglie Said Light & Elementary Particles Show both Properties
Feb 20, 2008
Nature of Light
Is light a wave or a particle? This question vexed physicists. for centuries. Huygens thought light was a type of wave. Newton thought light was made of a large number of particles. Diffraction effects, such as Young's double slit experiment seemed to prove that light was a wave. In the 19th century James Clerk Maxwell derived fundamental equations describing all electromagnetic phenomena. Maxwell's equations provided the theoretical underpinning showing that light was an electromagnetic wave.
In 1905 Einstein explained the photoelectric effect by postulating that light came in discrete quanta or chunks called photons. Einstein's explanation essentially proved that light was a particle.
So is light a wave or a particle? It can't be both, or can it?
Wave Particle Duality
In 1924, Louis-Victor De Broglie provided the answer with his hypothesis of wave particle duality. Is light a wave or a particle? Yes! It is a wave, and it is a particle.
Light exhibits both wave like and particle like properties.
Experiments such as Young's double slit experiment and other experiments that depend on diffraction of light waves or on constructive and destructive interference of light waves will show that light has wave like properties. Other experiments, such as the photoelectric effect, that depend on light being particles will show that light has particle like properties.
As strange as it may seem, light is both a wave and a particle. Experiments designed to prove it is a wave will work and show its wave like properties. In addition, experiments designed to prove that light is a particle will also work and show its particle like properties. According to De Broglie's wave particle duality hypothesis light has properties of both.
Wave Nature of Particles
De Broglie further extended his hypothesis to apply to elementary particle such as electrons, protons, and so forth. These particles, which physicists had always thought of as particles only, also display wave like properties. Just as for light, an experiment designed to prove that electrons (or other subatomic particles) are indeed particles will work and show their particle nature. An experiment designed to prove that they are waves will also work and show their wave like nature.
Wave particle duality works for things like light that physicists traditionally thought of as waves and for things like electrons and protons that physicists traditionally thought of as particles.
Davisson Germer Experiment
In 1927, Clinton Davisson and Lester Germer did an experiment that involved bouncing electrons of a nickel surface. The nickel atoms scattered the electrons in a pattern reminiscent of the light pattern produced by Young's double slit experiment. The nickel atoms and the spaces between them acted like a large number of slits that diffracted the electrons and produced a pattern of constructive and destructive interference. Diffraction and interference effects only occur for waves. The Davisson Germer experiment therefore confirmed De Broglie's hypothesis that particles also have wave like properties. Wave particle duality has since become a cornerstone of quantum physics.
De Broglie's brilliant solution: Just say yes - to both.
Further Reading
Knight, R.D., Physics for Scientists and Engineers with Modern Physics, Pearson, 2004.
Saxon, D.S., Elementary Quantum Mechanics, Holden-Day, 1968.
Heisenberg Uncertainty Principle
